Exceptional ceramic parts having laser optical transparency have been fabricated and are commercially available. These parts are typically made starting with a very pure co-precipitated powder which is then slip cast to form the green structure or preform prior to sintering. After vacuum sintering and hot isostatic pressing and polishing, the parts are rendered transparent enough for use in laser applications.
A uniform slurry of high purity powder is poured into a plaster mold which sucks the water out of the slurry by capillary forces and produces the green structure after drying. Using fluid flow and surface tension to consolidate the ceramic powder allows parts to be made with a uniform powder packing. However, because the mold removes the water, slip casting can only be used for relatively thin parts. The need for a very porous surface on the mold also introduces another variable in the green structure fabrication. The porous mold usually made of commercial gypsum may also be a source of contamination. Cold uniaxial pressing and cold isostatic pressing have also been used to make transparent parts. However, inter-particle friction during the pressing process tends to prevent densification in the center of the part so that size of the part must be kept small enough that this does not cause porosity.
In order to increase the driving force for sintering, a finer nano-sized powder than that produced by precipitation may be used. This can be especially important for achieving high transparency needed for lasers. Finer particles because of their increased surface area sinter more easily. Very small trapped pores are also less effective in scattering light.
However, smaller nano-sized particles behave differently than larger (such as micrometer) sized particles during green structure consolidation. For instance, smaller particles experience more friction as they move past one another in a die making it more difficult to produce a uniform structure through cold pressing, especially larger parts are desired. The higher surface area of finer particles also requires more water for wetting making it difficult to get the solids loading high for slipcast slurries. As a result, after slip casting there is significant shrinkage on drying often leading to cracking and other problems. Finer particles are more susceptible to surface-area-dependent chemical reactions, as may occur between a porous mold and certain ceramic powders.
Compound transparent ceramic optical components have been typically made by partially sintering one part, for example, the amplifier slab. Then the edges of the slab are carefully polished to a fraction of wavelength in flatness. A second part, a piece of a frame designed to suppress amplified spontaneous emission is similarly partially sintered and polished on its edge. The two pieces are placed in contact and held firmly in place in a furnace. Diffusion bonding occurs in which some atoms on one piece diffuse across the interface and into the other and the two pieces become bonded with no gap between them. This approach was used by Konoshima Ltd. to clad Nd:YAG amplifier slabs with an amplified spontaneous emission suppressing Sm:YAG frame (see Soules, Thomas F., “Transparent ceramics Spark Laser Advances”, Lawrence Livermore Science and Technology, April, 2006, available at www.llnl.qovlstrIAori/061Soules.html.).
Generally, parts may not be made with graded dopant concentrations, although such parts could be very beneficial to laser performance. Also, more complicated transparent ceramic compound structures than the one described above may not generally be made. One reason for this is that the above method employed by Konoshima would be difficult and expensive to extend to a complex part with many components much less a continuously varying composition.
Therefore, a method to more easily fabricate compound transparent ceramic structures, such as producing compound and complicated ceramic performs prior to sintering, would be very beneficial to the field of lasers.